3.1 Characterization of photocatalysts
The prepared impregnated catalysts (Cu/ZnO and Ni/ZnO) were characterized using SEM, EDX, XRD, Surface area. SEM was used to study the physical nature and surface morphology of copper and nickel impregnated ZnO and ZnO support to confirm that the copper and nickel after impregnation were significantly dispersed throughout the surface of ZnO. The morphology of (Cu/ZnO and Ni/ZnO) and ZnO are shown in Fig. 1a, b and c. The dispersed and mosaic nature of impregnated Cu/ZnO and Ni/ZnO as compared to bare ZnO shows that active metals Cu and Ni are successfully impregnated on the surface of zinc oxide 13.
The Electron Dispersive X-ray EDX spectra for Cu/ZnO, Ni/ZnO and ZnO are given in Fig. 2a, b and c respectively. Spectra shows concentration of Cu and Ni depicts that copper and nickel is successfully impregnated on ZnO 14.
The X-ray diffraction patterns of ZnO, Cu/ZnO and Ni/ZnO are given in Fig. 2d, XRD of pure ZnO with ICCD number 11136, 30888, 361451 and 11244 and shows peak at 28°,31°,32°, 34°, 36°, 47°,50°, 56°, 62°,66°, 67° and 69°. Figure 2d (b) is XRD of Cu/ZnO and its pattern according to ICDD number 11136, 30879, 30981, 50661 and 50664 whereas Cu shows peak at 43.6°, 50.7° and 74.4. The XRD diffraction pattern of Ni/ZnO are given in Fig. 2d (c). The XRD micrograph of Ni/ZnO showing pattern ICCD number 30888, 30891, 50664, 211486, 471019 and 11025. In these Figures, the corresponding peaks at angle greater than 30° show residue peaks that might have come from any residue present in water. It is observed that 2Ѳ values for the major reflections of Cu/ZnO is in range of 28° to 74.4° while for ZnO ranges from 28° to 69°. Surface area, band gap and FTIR of ZnO, Cu/ZnO and Ni/ZnO are reported in our previous work 12b.
3.2. Effect of pH
The solution pH is important in the process of ultrasound assisted photocatalytic degradation of herbicides. The influence of pH on ultrasound assisted photocatalytic degradation of isoproturon was studied by varying the solution pH from 2 to 10 using 0.1 g of photocatalysts (Cu/ZnO and Ni/ZnO), irradiation time (30 min) and herbicide solution (100 µg/ mL) in the presence of sonication and visible light. pH was adjusted using Britton- Robinson buffer. The effect of pH on % degradation of isoproturon by Ni/ZnO and Cu/ZnO was investigated (Fig. 3a). The results indicate that % degradation of isoproturon increases with increase in pH and reach to its maximum value of 80% at pH 7 using Ni/ZnO and and 76% using Cu/ZnO respectively. The influence of pH on % photocatalytic degradation of traisulfuron by Cu/ZnO and Ni/ZnO was also investigated from pH 2 to pH 10, (Fig. 3b). The results showed that % degradation of triasulfuron increases with increase in pH reaching to its maximum value (75% and 73%) at pH 7 and pH 6 using Cu/ZnO and Ni/ZnO respectively. It can be explained that at lower pH, Catalyst has positive surface which resist substrate and the concentration of H + is high that competes with herbicides for active sites leading to reduction in percentage degradation. The ultrasound assisted photocatalytic degradation decreases when pH increases from 8 to 12 decrease in % degradation at higher pH is due to negatively charged hydroxyl anion which inhibit free radicals in solution and active site of catalyst. As well as non-ionic nature of isoproturon are also responsible for maximum degradation of isoproturon at neutral pH 15. The increase in degradation on basic side as compared to acidic side is mainly due to attack of free radicals on methyl group of herbicides 16.
3.3. Effect of irradiation time
The time necessary for the ultrasound assisted photocatalytic treatment of herbicides was studied based on irradiation and sonication of the mixture. The irradiation time was gradually changed from 5 minutes to 60 minutes under the visible light in the sonicator by loading Cu/ZnO and Ni/ZnO impregnated photocatalyst (0.1 g) into isoproturon and triasulfuron (100 µg/ mL) sample solutions. The sonication passes ultrasound waves from the solution, increases the flow of solution to photocatalysts surface and homogenously distribute it within the solution. The effect of time on photocatalytic degradation efficiency of isoproturon under visible light is given in (Fig. 3c). The considerable increase in photocatalytic degradation of isoproturon with increase in sonication and illumination time may be explained as the sonication helps in mixing of substrate with photocatalysts and brings the herbicides into contact with catalyst resulting into increase in degradation efficiency. Maximum degradation of 83% and 78% was observed at optimum time of 25 min and 30 min using Cu/ZnO and Ni/ZnO respectively. The degradation efficiency was seen to decrease beyond the optimum time. Due to prolong irradiation and sonication the impregnated metals get discharged from the surface of ZnO catalyst and as a result its efficiency decreases. Results of the degradation of triasulfuron containing 0.1 g catalyst, at neutral pH and varying irradiation time, are given in Fig. 3d. The results suggest greater degradation with increasing irradiation time and reaches to maximum value (85% and 79%) at 25 minutes for Cu/ZnO and Ni/ZnO respectively. Further increases in time above the optimum value has no significant effect on degradation process. But it decreases with more sonication and illumination time 17.
3.4 Effect of photocatalyst dose
For economic removal of herbicides from the waste water the effect of photocatalysts dosage on photocatalytic degradation of herbicides was studied using impregnated Cu/ZnO and Ni/ZnO photocatalysts. Experiments were performed by varying the amount of photocatalysts form 0.02 g to 0.3 g keeping all other experimental conditions constant. Initially the photocatalytic activity increases with the amount of catalyst reaches to maximum degradation 86% and 80% using 0.1 g of Cu/ZnO and Ni/ZnO for isoproturon photocatalytic degradation respectively (Fig. 4a). The increase in catalyst amount beyond the optimum values does not significantly increase the efficiency of photocatalytic degradation. The photocatalytic degradation was slightly reduced beyond the 0.2 g to 0.3 g of photocatalysts, this is due to large amount of photocatalyst amount in solution leads to light scattering by the particles, less light penetration through the herbicides solution and reduction in transparency of aqueous medium. The degradation of triasulfuron with varying amount of Cu/ZnO and Ni/ZnO catalyst was also investigated and results are given in Fig. 4b. There is increase in degradation of triasulfuron with catalyst dosage until the maximum photocatalytic decomposition 89% and 86% is obtained for Cu/ZnO (0.08 g) and Ni/ZnO (0.1 g). The degradation slightly decreases beyond 0.15 to 0.30 g of catalyst, explained as greater amount of catalyst in solution causes light scattering, reduction in solution transparency and less light penetration 15.
3.5. Effect of oxidants
Different oxidants such as potassium persulfate, sodium perchlorate and hydrogen peroxide were applied in concentration range of 1 Mm to 6 Mm and its effect on ultrasound assisted photocatalytic degradation was studied. Experimental work was performed by varying the concentration of enhancers keeping optimised pH, catalyst dosage and concentration of herbicides for 30 minutes. The sample was placed in dark to attain equilibrium and then placed in sonicator under tungsten filament lamp and after each 05 minutes known concentration was taken, absorbance was noted and percent degradation was calculated. The effect of oxidising agents on the degradation of isoproturon was determined and 81% degradation was observed using Cu/ZnO for 4 mM of K2S2O8, 89% degradation for 4 mM of NaClO4 and 93% for 3 mM of H2O2. Using Ni/ZnO photocatalyst 83% degradation was observed with 5mM of K2S2O8, 87% degradation with 3 mM of NaClO4 and 91% degradation with 4mM of H2O2 was observed (Fig. 5a, b and c). The influence of addition of oxidants on ultrasound assisted photocatalytic degradation of triasulfuron was also studied and the results are presented in (Fig. 5e, f and g). It was shown that degradation increases with oxidant due to more radical’s formation 13b. As these free radicals are responsible for the degradation so by increasing the concentration of oxidising agents the generation of free radicals increases but after optimum value the degradation efficiency decreases due to inhibition of radicals that was due to interaction of radicals with each other. 5mM of potassium persulfate concentration was found to be optimum for triasulfuron and 88% and 82% degradation was found using Cu/ZnO and Ni/ZnO visible light induced photocatalysts respectively (Fig. 5d). Effect of sodium perchlorate as an enhancer was also studied and with 5 mM of NaClO4 93% degradation was found using Cu/ZnO as photocatalyst and with 4mM of NaClO4 86% degradation was observed using Ni/ZnO for triasulfuron (Fig. 5e). Hydrogen peroxide effect as an enhancer was also investigated and 93% and 91% degradation was observed with 5 mM of H2O2 using Cu/ZnO and Ni/ZnO (Fig. 5f).
3.6. Effect of herbicide concentration
The effect of herbicide concentration on ultrasound assisted photocatalytic degradation of isoproturon and triasulfuron was studied in the range of 5 ug/mL to 50 ug/mL using Cu/ZnO and Ni/ZnO (Fig. 6). Experimental work was performed using pH 7 for isoproturon and pH 7 and pH 6 using Cu/ZnO and Ni/ZnO for triasulfuron, 3mM of H2O2 using Cu/ZnO and 4mM of H2O2 using Ni/ZnO for isoproturon and 5mM of H2O2 using Cu/ZnO and Ni/ZnO for triasulfuron, 0.1 g of photocatalysts (Cu/ZnO and Ni/ZnO) for irradiation time of 30 minutes and varying herbicides concentration in the presence of sonication and visible light. The results showed that 99% degradation of 5 ug/mL isoproturon was achieved in presence of Cu/ZnO and 98% degradation in presence of Ni/ZnO. While 98% and 99% ultrasonic assisted photocatalytic degradation of 5 ug/mL triasulfuron was achieved in presence of Cu/ZnO and Ni/ZnO photocatalysts respectively. The degradation decreases with increase in concentration. With 50 ug/mL the degradation of isoproturon decreases with increase in concentration and it was found to be 91% and 88% with Cu/ZnO and Ni/ZnO while in case of 50 ug/mL concentration of triasulfuron the degradation efficiency decreases to 91% and 87% with Cu/ZnO and Ni/ZnO photocatalysts respectively. The decrease is explained by the enhanced light scattering with increase in herbicide concentration and decrease in the concentration of free radicals, as these OH° are responsible for the degradation of herbicides 18.
3.7. Effects of interferences
The influence of the presence of other chemical ions on degradation efficiency of isoproturon and triasulfuron is necessary to study its interaction with the degradation mechanism. The effect of commonly present ions such as sulfates, carbonates and chlorides on the photocatalytic activity was studied by adding the variable amounts of ions in concentration range of (1 mM to 5 mM) to reaction mixture, while keeping all other experimental conditions that is pH 7 for isoproturon and pH 7 and pH 6 using Cu/ZnO and Ni/ZnO for triasulfuron, 3mM of H2O2 using Cu/ZnO and 4mM of H2O2 using Ni/ZnO for isoproturon and 5mM of H2O2 using Cu/ZnO and Ni/ZnO for triasulfuron, Catalyst amount (0.1 g Cu/ZnO and Ni/ZnO), irradiation time (30 minutes), herbicides concentration (100 µg/ mL) as constant. The results showed that isoproturon degradation decreases to 73% with 6mM of SO42− using Cu/ZnO and Ni/ZnO photocatalysts (Fig. 7a), 75% and 74% using 6mM of CO32− in the presence of Cu/ZnO and Ni/ZnO photocatalysts (Fig. 7b) and 82% and 81% using 6mM of Cl− in the presence of Cu/ZnO and Ni/ZnO photocatalysts (Fig. 7c). While in case of triasulfuron the ultrasound assisted photocatalytic degradation was also effected both by concentration and size of the added diverse ions. It is explained that diverse ions diminish the free radicals formed in the photocatalytic process due to its larger size and grater charge it slow down the photocatalytic degradation process 8. The degradation of triasulfuron decreases to to 75% and 71% with 6mM of SO42− using Cu/ZnO and Ni/ZnO photocatalysts (Fig. 7d), 76% and 78% using 6mM of CO32− in the presence of Cu/ZnO and Ni/ZnO photocatalysts (Fig. 7e) and 86% and 82% using 6mM of Cl− in the presence of Cu/ZnO and Ni/ZnO photocatalysts (Fig. 7f).
3.8 Recovery and re usability
After photocatalytic treatment, the recovery and reusability of the photocatalysts were checked. The ease of reuse and good recovery after photocatalytic activity is of great economic advantage, the used photocatalysts was recovered by filtration, followed by washing, drying and calcination in the oven. The photocatalysts were washed with distilled water and then with organic solvents such as ethanol and acetone. After grinding the recovered impregnated zinc oxide photocatalysts were ready for reuse. The ultrasound assisted photocatalytic degradation efficiency of first, second and third time recycled photocatalysts using isoproturon are given in Fig. 8a. The percent degradation of Cu/ZnO are 93%, 92% and 90% and the degradation efficiency of Ni/ZnO are 92%, 91% and 89% for isoproturon degradation after once, twice and thrice photodegradation. It was noted that the photocatalytic activity of recycled photocatalyst are very close to photocatalytic activity of fresh photocatalyst. Thus Cu/ZnO and Ni/ZnO can be recycled without any compromise on photocatalytic activity. The sonication assisted photodecomposition of triasulfuron once, twice and thrice recycled catalyst are shown in Fig. 8b. The percent degradation of Cu/ZnO are 97%, 95% and 93% and of Ni/ZnO are 95%, 93% and 91% for triasulfuron photocatalytic degradation after first, second and third use of the catalyst. The results shows no significant decrease in degradation. The proposed method is best for commercial applications keeping in mind the reusability of photocatalyst. The prepared photocatalysts were compared with other catalysts and degradation methods (Tables 1, 2 and 3). The comparison of results revels that proposed ultrasonic assisted photocatalytic degradation method of isoproturon and triasulfuron is best as compared to other methods. The plausible degradation mechanism of isoproturon are given in Fig. 9.
Table 1
Comparison of the proposed degradation method with other degradation methods used for Isoproturon
No. | Herbicide | Degradation Source for | Catalyst | Time (min) | % Degradation | References |
---|
1. | Isoproturon | Sonophotocatalytic | TiO2 P25 with that of Kronos 1077 | 240 and 60 min Respectively | 100 | 19 |
2. | Isoproturon | Photocatalytic | TiO2 | 360 | 85 | 20 |
3. | Isoproturon | Photocatalytic | TiO2 over H-mordenite (H-MOR) | 300 | 80 | 21 |
4. | Isoproturon | Photocatalytic | Activated carbon (AC) and titania catalyst (TiEt) | 1920 | 56 | 16 |
5. | Isoproturon | Solar PhotoFenton | H2O2 | 150 | 100 | 22 |
6. | Isoproturon | Photocatalytic | TiO2/Al-MCM-41 | 90 | 80 | 18 |
7. | Isoproturon | Photocatalytic | TiO2 (Degussa P25) | 80 | 94.8 | 23 |
8. | Isoproturon | Sonophotocatalytic | Cu/ZnO and Ni/ZnO | 30 | 99 | Present work |
Table 2
Comparison of the proposed degradation method with other degradation methods used for Triasulfuron
No. | Herbicide | Degradation Source for | Catalyst | Time (min) | % Degradation | References |
---|
1. | Triasulfuron | Photocatalytic | ZnO/Na2S2O8 | 120 | 100 | 24 |
2. | Triasulfuron | Photodegradation | UV | 6000 | 60 | 25 |
3. | Triasulfuron | Photocatalytic | TiO2 | 320 | 99 | 26 |
9. | Triasulfuron | Sonophotocatalytic | Cu/ZnO and Ni/ZnO | 30 | 98 | Present work |
Table 3
Degradations of different herbicides by Cu/ZnO and Ni/ZnO
No. | Herbicide | Degradation Source for | Catalyst | Time (min) | % Degradation | References |
---|
1. | organophosphate chlorpyrifos | Photocatalytic | Cu-ZnO | 240 | 85 | 27 |
2. | Bentazon | Photocatalytic | Cu-ZnO | 60 | 98.28 | 28 |
3. | MB Dye | Photocatalytic | 4% Cu–ZnO (CZS-25 NCs) | 60 | 100 | 29 |
4. | Lindane | Photocatalytic | Zn@ZnO | 40 | 99.5 | 30 |
5. | Diazinon | Photocatalytic | Ni-ZnO | 120 | 99.96 | 31 |
6. | imidacloprid | Photocatalytic | PANI/ZnO-CoMoO4 | 180 | 97 | 32 |
7. | P-nitrophenol | Photocatalytic | Ni-ZnO/Ppy | 180 | 96.04 | 33 |
8. | ciprofloxacin | Photocatalytic | Ni-ZnO | 90 | 83.7 | 34 |
9 | Dicofol | Photocatalytic | Ni-ZnO | 90 | 99.2 | 35 |
10. | Isoproturon and Triasulfuron | Sonophotocatalytic | Cu/ZnO and Ni/ZnO | 30 | 99 & 98 | Present work |